Coatings for refractory-metalbase alloys



1970 N. s. BORNSTEIN ETAL 3,489,535

COATINGS FOR REFRACTORY-METAL-BASE ALLOYS Filed Oct. 5, 1966 3Sheets-Sheet 1 FIG. 1

Sn RICH ENVELOPE ZONE Cb AI3 RICH ZONE Cb Sn RICH ZONE SUBALUMINIDE ZONECb-IZr ALLOY SUBSTRATE PHOTOMICROGRAPH OF COATED Cb-lZr ALLOY SUBSTRATEIN AS COATED CONDITION FIG. 2

Sn RICH ENVELOPE ZONE CbAI3 RICH ZONE Cb Sn RICH ZONE SUBALUMINIDE ZONECb-IZr ALLOY SUBSTRATE 500x PHOTOMICROGRAPH OF COATED CbiZrALLOYSUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR 100 HOURS INVENTOIBNORMAN S. BORNSTEIN LEONARD A. FRIEDRICH EMANUEL C. HIRAKIS ATTORNEYSJan. 13, 1970 s. BORNSICIN T AL 3,489,535

COATINGS FOR REFRACTOHYIBTALBASE ALLOYS Filed Oct. 5, 1966 5 Sheets-Shea2 FIG. 3

Sn RICH ENVELOPE ZONE Cb m RICH ZONE Cb Sn RICH ZONE SUBALUMIN'DE ZONECb-iZr ALLOY SUBSTRATE PHOTOMICROGRAPH OF COATED Cb-1Zr ALLOY SUBSTRATEAFTER EXPOSURE IN ARGON AT 2000F FOR 500 HOURS Cb Ala RICH ZONEJ Cb SnRICH ZONE SUBALUMINIDE ZONE? Cb-lZr ALLOY SUBSTRATE 50C! PHOTOMICROGRAPHOF COATED Cb-iZr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR1000 HOURS INVENTORS NORMAN S. BORNSTEIN LEONARD A. FRlEDRICH EMANUEL C.HIRAKIS ATTORNEYS Jan. 13, 1970 5. o s'r m ET AL 3,489,535

COATINGS FOR REFRACTORY-METAL-BASE ALLOYS Filed Oct. 5, 1966 A 3Sheets-Sheet 3 FIG. 5

Cb Al RICH ZONE Cb Sn RICH ZONE SUBALUIVIINIDE ZONE Cb-i Zr ALLOYSUBSTRATE 500x PHOTOMICROGRARH OF COATED Cb-lZr ALLOY SUBSTRATE AFTEREXPOSURE IN ARGON AT 2000F FOR 2500 HOURS Cb A13 RICH ZONE Cb3Sn RICHZONE SUBALUMINIDE ZONE Cb--1 Zr ALLOY SUBSTRATE PHOTOIVIICROGRAPH OFCOATED Cb-iZr ALLOY SUBSTRATE AFTER EXPOSURE IN ARGON AT 2000F FOR SOOOHOURS INVENTORS NORMAN S. BORNSTEIN LEONARD A. FRIEDRICH EMANUEL C.HIRAKIS ATTORNEYS United States Patent Ofice 3,489,535 Patented Jan. 13,1970 US. Cl. 29194 17 Claims ABSTRACT OF THE DISCLOSURE Oxidation andcontamination protective coatings are provided for columbium, tantalum,molybdenum, tungsten, and alloys of these refractory metals. Thecoatings have an exterior coating zone consisting essentially of from 40to 50% Sn, from 27 to 33% Cr. from 14 to 18% Al, and from 7 to 11% Ti;and an interior coating zone, located between the exterior coating zoneand the refractory metal substrate, consisting essentially of 65 to 75%Sn, 11 to 15% Al, 7 to 11% Ti, 2 to 6% Cr, to 4% Zn, and from 0 to 3% ofan alkali or alkaline earth metal halide. Both coating zones aremodified during their bonding to the substrate and in subsequent heattreatment and use by the diffusion of the refractory metal of thesubstrate into the coating zones. These coatings afford long timeoxidation and contamination protection to refractory metal substrates atintermediate temperatures up to about 2000 F.

This invention relates to coatings for the refractory metals and theiralloys that will protect such metals from atmospheric contamination athigh temperatures.

More particularly this invention relates to two-zone thermally andmechanically stable coatings for the refractory metals that will protectsuch metals from atmospheric contamination for long periods of time inintermediate temperature environments.

The coatings of this invention are designed to protect refractory metalsubstrates at temperatures of from about room temperature up to at leastabout 2000 F. Although these coatings are primarily designed for use inprotecting geometrically complex engineering structures and assembliesmade from the refractory metals and their alloys, they are alsoparticularly useful in coating laboratory test specimens of refractorymetals.

As used in this specification and claims, the term refractory metalsrefers to those nonprecious refractory metals having melting pointsequal to or higher than the melting point of chromium (Cr), or 3407 F.(1875 C.). So defined, the refractory metals of this application inascending order of their melting points are thus: chromium (Cr),vanadium (V), hafnium (Hf), columbium (Cb), molybdenum (Mo), tantalum(Ta), and tungsten (W). The term refractory metals as used herein alsorefers to alloys having refractory-metal bases, as well as to therefractory metals themselves. The invention in its most importantaspects relates to protective coatings for Cb-base substrates.

For many years it has been generally known that the high-temperaturestrength properties of metals are closely related to their meltingpoints. In general, metals having high melting points thus are capableof forming alloys having high strength at high temperatures. In recentyears, the need for new structural materials for service at temperaturesin excess of those that can be withstood by conventional structuralmaterials has stimulated interest in those metals having the highestmelting points, or the refractory metals and their alloys.

As alloy base materials for high-temperature service, a number of thesemetals have shown much promise in various high-temperature applications.Perhaps one of the most versatile and promising of these metals is Cband considerable work has been done to develop it as a structural alloybase for uses in high-temperature environments.

Among the technically more important physical qualities of Cb as analloy base are its high melting temperature (4474 F.) and its lowneutron-capture cross-section. Further, Cb is inherently a soft,ductile, readily fabricable metal and, although it becomes too weak forpractical structural uses at temperatures much above 1200 F., it iscapable of being strengthened for use at much higher temperatures byalloying it with various other metals, particularly with otherrefractory metals. Disadvantageously, Cb is a highly reactive metal atelevated temperatures and will dissolve relatively large quantities ofnitrogen and oxygen on exposure to atmospheres containing even smallamounts of these elements at moderately elevated temperatures.

Because of the relative importance of Cb, much of the description thatfollows is based on the use of the coatings of this invention on Cb orCb-alloy substrates. It will be understood, however, that the scope ofthe invention is not limited to coatings for Cb-base substrates, butincludes coatings for the refractory metals generally. To some extent,the nature of the substrate, particularly as governed by the primary orpreponderant element present, determines the formulation of the coatingof this invention that is most effective for the substrate in question.

It is well known that no metal is completely resistant to surfacecontamination from exposure to air at elevated temperatures. Most metalsthat can be used at high temperatures without surface protection form athin adherent protective oxide coating during initial exposure. Thisoxide coating insulates the base metal from further oxidation as long asit remains intact. The pure metals and alloys that exhibit thisattribute of self-protection are, however, generally limited in theiruse to temperatures below about 1800 F.

The refractory metals and their alloys are essentially the only metalsthat retain suflicient strength at temperatures above about 1800 F. tomake them useful at these temperatures. In recent years the refractorymetals have been subjected to extensive study, investigation, anddevelopment. Various of the refractory metals that are available insufficiently abundant supply for possible commercial development havebeen evaluated for numerous high-temperature uses. Unfortunately, noneof the refractory metals has sufiicient resistance to oxidation orcontamination in air at high temperatures to be used without protection.

The refractory metals do not form their own adherent and protectivecoatings within the temperature ranges of primary interest for theiruses. Many of the most promising of these metals, such as Cb. Ta, and Moare subject to extremely rapid or even catastrophic oxidation ifunprotected in air at temperatures above 1000 F. Such oxidation vitiatesand destroys the high-temperature strength of these metals. Accordingly,many efforts have been directed toward forming effective coatings forthe refractory metals which inhibit or prevent their oxidation andcontamination at high temperatures.

In the past, most of these efiorts have been directed to the productionof coatings suitable for oxidation protection of the refractory metalsat extreme temperatures and short exposure times, such as those whichwould be encountered by composite structures on atmospheric reentry.Very little effort has been devoted to the production of thermally andmechanically stable protective coatings that will provide long-timeprotection for refractory metal substrates at intermediate temperatureenvironments, such as from room temperature up to about 2000" F.Coatings of this character are of particular utility in providingprotection for test speciments being used for laboratory testing anddevelopment of refractory metal structures.

In the past, the absence of a protective coating suitable for long-time,intermediate temperature protection has required that such labotatorspecimens be tested in highly purified inert cover gas or extremely highvacuum (on the order of torr) test environments. The provision of suchtesting environments not only requires greatly increased testingexpenditures, but also decreases the reliability of the specimens, andhence the value of the test results.

It is obvious that coatings which provide long-time, intermediatetemperature protection also would be useful for many applications whererefractory metal substrates are to be subjected to such conditions inuse. An example of such a requirement would be in oxidation protectivecoatings for structural parts of advanced nuclear power plants.

The requirement that refractory metals be tested under an inertatmosphere, due to their oxidation-prone character, is discussed above.However, even when a controlled protective atmosphere is used,unacceptable contamination can result. In certain uses of the refractorymetals, the presence of even very small amounts of oxygen in the basemetal can have serious deleterious results, even though the strength ofthe metal members remains substantially unimpaired. This is particularlytrue when a structural member is used for containment of liquid metals.For example, Cb-alloys, because of their relative strength,availability, and fabricability are outstanding candidates as structuralmaterials for liquid metal containment. Other refractory metals havealso displayed favorable compatibility with alkali liquid metals, suchas lithium (Li).

Pure Cb shows no susceptibility to solution attack by purified Li attemperatures up to 2200 F. However, when the oxygen in solution in Cbreaches a concentration of as little as a few hundred parts per million,Cb may be rendered sensitive to intergranular Li attack. Under theseconditions Li will penetrate grain boundaries of Cb-base alloys andactually seep through the metal. The attack occurs at all temperaturesabove 1000 F. and is quite rapid, reaching completion in a few minutes.

While this susceptibility to Li attack in Cb may be reduced by alloyingthe Cb with zirconium (Zr) and heat treating, the Cb will remainsusceptible to Li attack if oxygen atoms are present in amounts inexcess of twice the Zr atoms present. Accordingly, when Cb or otherrefractory alloy substrates are desired to be used as structuralmaterials for containment of liquid alkali metals, it is of the utmostimportance that the refractory metals, and particularly Cb, be protectedfrom oxidation at all stages of their manufacture. Thus, where theseuses are contemplated, it is particularly important that the oxidationresistant coatings of this invention be used, even if fabrication is tobe carried out largely or completely under inert environments.

Various types of coatings have been provided for Cband other refractorymetal-base substrates in the prior art. For example, certain silicideand aluminide coatings have been used. Exemplary of the latter type arealuminum-silicon (such as Al-lOSi) and tin-aluminum (such as Sn-lOAl)coatings. However, none of these coatings have provided satisfactorylong-time oxidation protection for the substrates at the intermediatetemperatures needed for the requirements and uses described above.

In view of the foregoing, it is a primary object of this invention toprovide new and improved protective coatings for refractory metalsubstrates at intermediate temperatures, whereby such substrates can besubjected to exposure to air at elevated temperatures up to ab ut 2000F. for long periods of time without danger of oxidation orcontamination, and to provide composite articles having refractory metalsubstrates and such new and improved protective coatings.

Another object of this invention is to provide new and improvedthermally and mechanically stable coatings for refractory metalsubstrates which provide excellent p tection against oxidation andcontamination of such substrates during their subjection to intermediatetemperatures, of from about room temperature up to about 2000 F., forlong periods of time.

Another object of this invention is to provide a suitable protectivecoating for refractory metal test specimens, to protect such specimensfrom oxidation during long-time testing in both impure inert andoxidative atmospheres.

Still another object of this invention is to provide new and improvedtwo-zone coatings for refractory metal substrates that will protect thesubstrates from oxidation for long periods of time at intermediatetemperatures up to about 2000 F.

Yet another object of this invention is to provide a two-zone coatingfor refractory metal substrates, each coating zone of which containscritical amounts of Al, Sn, Cr, and Ti.

A still further object of this invention is to provide improved coatingsfor refractory metal substrates that will protect the substrates fromoxidation and contamination at intermediate temperatures for longperiods of time, the coating having a self-healing Sn-containing surfacezone which prevents contamination or oxidation of the u strate bycracking of the coatings or the formation of defects in the coatings.

Another object of this invention is to provide improved them fromcontamination during long exposure to air at intermediate temperaturesup to about 2000 F. with a coating having a self-healing metallicsubsurface zone backed up by a self-healing metallic surface zone inwhich the two zones cooperate together to prevent contamination oroxidation of substrates by cracking or formation of other defects in thecoatings.

Yet another object of this invention is to provide a coating forrefractory metal substrates that can be readily applied to bothlaboratory specimens and dimensionally large and geometrically complexengineering structures without sacrificing coating performance. Thesecoatings are amenable to application and repairs in the field, and canbe applied uniformly both in thickness and in composition over theentire surface of the substrate.

Additional objects and advantages will be set forth in part in thedescription that follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention, theobjects and advantages being realized and attained by means of thecompositions, methods, and processes particularly pointed out in theappended claims.

To achieve the foregoing objects and in accordance with its purpose,this invention includes, as broadly described, a coated metal articlehaving a substrate selected from the group consisting of the refractorymetals and alloys thereof, and a coating having, in the as-a plied form:

(1) An exterior or surface zone consisting essentially of certaincritical amounts of Sn, Al, Cr and Ti, and

(2) An interior coating zone interposed between the refractory metalsubstrate and the surface or exterior zone of the coating, describedabove, which also consists essentiall of certain critical amounts of Sn,Al, Cr, and Ti.

In accordance with the invention, upon heat treatment, the primaryelement or elements comprising the substrate alloy diffuse outwardlyinto both the interior and exterior coating zones and become elementalmodifying constituenls of the coating itself. For example. if a Cb-basesubstrate is Coated according to the invention, Cb will diffuse intoboth coating zones during heat treatment and will become part of theresultant coating composition. Similarly, if a Cb-base alloy substratecontaining given amounts of Ta, W, and M0 is coated, Cb, Ta, W, and Mowill all be present in the coating after heat treatment and in amountsroughly proportional to the relative amounts of each of these elementsthat is present in the substrate alloy composition.

These elemental additions to the as-applied coating compositions thatoccur through the phenomenon of diffusion contribute to and enhance thebeneficial performance of the coating. The final or completed coatingsof this invention, as contrasted with the as-applied coatings, may thusbe described as substrate-modified.

In addition to Al, Sn, Cr, and Ti, the interior coating zone of thecoatings of this invention can contain, as optional ingredients, zinc(Zn) and an alkali metal halide or alkaline earth metal halideactivator.

In accordance with the invention, the interior or subsurface coatingzone as-applied consists essentially of from 65 to 75% by weight of Sn,from 11 to by weight of Al, from 7 to 11% by weight of Ti, from 2 to 6%by weight of Cr, from 0 to 4% by weight of Zn, and from 0 to 3% byweight of an alkali metal halide or alkaline earth metal halideactivator. Preferably, Zn when present is in an amount of from 1 to 4%by weight of the interior coating zone, and the halide activator ispresent in an amount of from 1 to 3% by weight of the interior coatingzone.

The exterior or surface zone of the coatings of this inventionas-applied consists essentially of from to by weight of Sn, from 27 to33% by weight of Cr, from 14 to 18% by weight of Al, and from 7 to 11%by weight of Ti.

An optimum coating in accordance with this invention has an interiorcoating zone as-applied consisting essentially of about: 70% Sn, 13% Al,9% Ti, 4% Cr, 2% Zn, and 2% HF; and an exterior coating zone as-appliedconsisting essentially of about: 45% Sn, 16% Al, 30% Cr, and 9% Ti.

The coatings of this invention are preferably applied to the surfaces ofrefractory metal articles by a spray or slurry deposition process.Generally, this process comprises:

(1) Forming an interior coating zone as-applied on the refractory metalsubstrate which consists essentially of Sn, Al, Cr, and Ti, and asoptional ingredients, Zn and the halide activator, all in the amountsset forth above; and

(2) Applying over the interior coating zone a second coatingcomposition, thereby forming an exterior or surface coating zoneas-applied on the composite consisting essentially of Sn, Al, Cr, andTi, all in the amounts set forth above.

First, following the application of the interior coating composition tothe refractory metal-base article, and again, following the applicationof the exterior coating composition, the composite is fired to apredetermined temperature to produce a uniform and adherent coating onthe substrate which is substantially impervious to gaseous contaminants,such as oxygen, at the intermediate elevated temperatures to which theproducts of this invention are intended to be subjected. During thesefirings some substrate-modification of the interior and exterior coatingzones occurs through diffusion of substrate elements into the coatingzones, as previously described.

Both of the coating zones of this invention are preferably applied tothe substrate, or previously coated substrate, by a cold spray slurryprocess. In this process the coating composition is dispersed in avaporizable diluent in an amount sufficient to give the composition asprayable consistency and then sprayed onto the surface of thesubstrate. Generally, a binding or sticking agent is included in thesuspension of the coating composition. The binding or sticking agentcauses particles of the coating composition to adhere both to each otherand to the substrate or the other coating composition previously appliedto the substrate, as the case may be.

While, as pointed out above, a cold spray slurry process is thepreferred method of applying the coating compositions of this invention,any other suitable methods, which will be readily apparent to thoseskilled in the art, can, of course, be used.

For ease of description in the bulk of the specification that followsthe coatings of this invention will be described specifically as theywould be used on a Cb-base substrate. Although this specific descriptionmay thus at times appear to apply only to use of the invention onCb-base substrates, it will be understood that it is not so limited andthat other refractory-metal-base substrates can be substituted for Cbwith substantially equivalent results to those obtained with the Cb-basesubstrates on which the specific description is based.

When a Cb-base substrate is used with coatings of this invention, the Alin the interior coating composition forms an oxidation resistantintermetallic columbium aluminide composition (CbAl with the Cb of thesubstrate. This columbium trialuminide (CbAl provides the primaryoxidation and contamination protection afforded by the interior coatingzone for protection of the refractory metal substrate and providesimportant oxidation resistance at intermediate temperatures up to about1800" F. To fulfill this important function of CbAl formation, A] ispresent in the interior coating zone as-applied in amounts of from 11 to15% by weight of the total coating of that zone.

Sn is also a primary component of the interior coating composition andit performs a variety of functions in the coating. Al has a limitedsolubility in Sn, and because Sn liquefies at a relatively lowtemperature, it performs the useful role of carrying amounts ofdissolved Al throughout the coating to bring Al into contact withunreacted or partially reacted Cb from the substrate thereby to form theoxidation resistant CbAl component of the coating.

This function of the Sn in the coating is important both in the initialformation of the CbAl and in elevated temperature service, whendiffusion of Cb from the substrate into the coating, or recession of thecoating into the substrate, can result in formation of subaluminideswhich do not have the oxidation resistance capacity of the desiredCb-trialuminide (CbAl The Sn is believed to transport Al throughout: thecoating to provide Al for reaction during service with excess Cb whichdiffuses into the coating at heat-treatment temperatures and usetemperatures, as previously described.

The Sn thus gives the coating self-healing properties, since itsdiffusion into a liquid phase with dissolved Al throughout the coatingprovides Al for the production of new Cb-trialuminides at any sites inthe coating where oxidation resistance may have been reduced byformation of subaluminides less oxidation resistant than CbAl Sn alsoforms an intermetallic compound with Cb from the substrate, Cb Sn, whichintermetallic compound is located between the substrate and the CbAllayer. This Cb Sn layer exhibits a coefficient of thermal expansionintermediate between that of the substrate and the main Cb-trialuminidecoating zone and therefore acts as a thermal shock absorber for thecoating.

To fulfill these varied and important functions, Sn is present in theinterior coating composition as-applied in amounts, by weight, of from65 to 75%. Less than 65% Sn in the coating is insuflicient to performthe foregoing functions, but the maximum permissible Sn content in theinterior coating zone is 75%, because more than this would not allow forthe requisite amounts of Al, Cr, and Ti in that coating zone.

The critical amounts of Ti and Cr that are present in both the interiorand exterior coating zones of the coatings of this invention produce thegreatly improved coating performance which is the essence of theinvention. The presence of Ti and Cr in the coatings effectivelycounteracts the low temperature aluminide pest phenomenoncharacteristics which are usually exhibited by Cb-trialuminide coatings(powdering at temperatures of about 1200 to 1600 F.) For these reasons,it is necessary that the interior coating compositions of this inventioncontain Ti and Cr in the amounts specified. Additionally, the presenceof these ingredients in the critical amounts specified results in thegreatly improved coating life afforded by the coatings of this inventionat all of the elevated temperatures contemplated by this invention, andparticularly at temperatures of about 1200 F., 1600 F. and 2000 F.,which are approximate temperatures representative of temperature rangesoften encountered in actual use of refractory metal alloys of the typewhich are beneficially coated in accordance with this invention.

Although applicants do not wish to be bound by any particular theory asto the reasons for the improvement resulting from the addition of Ti andCr to the coatings, it is believed that Ti replaces some Cb atoms andthat Cr replaces some Al atoms in the Cb-trialumimide latice structure.These substitutions substantially improve coating performance over thatnormally obtained with unmodified Cb-trialuminide coatings.Additionally, it is believed that Ti and Cr are both soluble to someextent in Cb-trialuminides. The final coating also probably containsminor amounts of Ti-aluminides and Cr-aluminides, since both Ti and Crform refractory aluminides. For one or more of these reasons,significant improvements in coating performance result from theinclusion of the specified amounts of Ti and Cr in the interior coatingzone. To achieve these highly benefical coating attributes, the Ticontent of the interior coating zone must be at least 7% and the Crcontent of the interior coating zone must be at least 2% in theas-applied condition. The presence of more than 11% by weight of Ti andmore than 6% Cr in the interior coating zone in the as-applied conditionproduces erratic coating performance and must be avoided, because thepresence of Ti or Cr in excess of the stated amounts causes difficultiesin bonding the exterior coating zone to the coated substrate.

In addition to the foregoing attributes of the coatings of thisinvention are the benefits resulting from the selfhealing properties ofthe coatings. In addition to Al, both Ti and Cr also have somesolubility in liquid Sn, and thus the liquid Sn acts to supply fiaws inthe coating with the reactive metals used-Al, Ti, and Cr. Until thesereactive materials are entirely oxidized or consumed by reaction withthe substrate or with orygen this self-healing mechanism of the coatingsof this invention continues.

The self-healing properties of the coatings imparted by liquid Sn(containing dissolved Al, Ti, and Cr) are useful in correcting defectssuch as cracks that may be present in the coating after its initialformation. If such cracks or other defects are present, initial thermalcycling of the coating in use will generally effect healing of thesedefects, through the self-healing properties imparted by liquid Sn.

Sn in liquid phase is believed to transport A] through the coating byconvection and gross carrying as well as through solution of Al in Sn.Because the liquid phase Sn carries Al and other particles to desiredreaction sites, it promotes formation of a uniform coating at bothminimum exposure times and minimum elevated temperatures.

One optional component of the interior coating composition is anactivating agent comprising an alkali metal halide or alkaline earthmetal halide. This activator is preferably present in the interiorcoating zone in the asapplied condition in amounts of from 1 to 3% byweight. The halide activating agent serves to flux the metal powdersused in the production of the interior coating zone, particularly theAl, and promotes coalescence, wetting, fusion, and reaction of the metalpowders to create the desired intermetallic composition.

The activating halide also serves to reduce oxide films on the metalpowder particles-particularly on Al-as the coated substrate is heated,thereby promoting the desired intermetallic reaction.

The optional metal component of the interior coating composition,namely, Zn-like Ti and Cr, discussed aboveis believed to achieve variousfunctions that improve coating performance, such as increasing long-termoxidation resistance, promoting self-healing properties, anddeoxidizing. However, this ingredient is truly optional and can beomitted entirely from the interior coating composition. Zn should not bepresent in the interior coating zone in the as-applied condition, in anycase, in an amount greater than 4% by weight, because at levels above 4%it can adversely affect the beneficial proporeties of the coating.

After the interior coating composition has been applied to thesubstrate, preferably by a cold spray slurry process, it is heat treatedat from 1800 to ZOO-0 P. for from 1 to 16 hours, and preferably for from1.5 to 4 hours. Optimum heat treatment is for 2 hours at 1950 F. Thisheat treatment produces an adherent interior coating zone on thesubstrate, formed from the interior coating composition. The interiorcoating composition is preferably applied to the article being coated inan amount of about 20 to 25 mg./cm. of surface area-optimum is about 23mgfcmF-but these amounts are not critical.

Following this heat treatment, the exterior or surface coatingcomposition consisting essentially of Sn, Cr, Al, and Ti is applied tothe previously coated composite. The Sn and Al in this exterior coatingcomposition provide a reservoir of excess amounts of these elements inthe coating which modify the Cb-aluminides formed in the interiorcoating zone. For these purposes the exterior coating composition in theas-applied condition contains to by weight of Sn and 14 to 18% by weightof Al.

The Ti and Cr in the exterior coating composition also modify theCb-aluminides of the second coating zone, and when present in thespecified amounts, provide greatly improved coating performance for thereasons discussed above. The exterior coating composition, as-applied,contains from 7 to 11% Ti and from 27 to 33% Cr.

It will be noted that the amounts of Cr present in the exterior coatingzone are substantially greater than the amounts of Cr present in theinterior coating composition. The incorporation of these greater amountsof Cr in the exterior coating zone is possible because of the separationof this exterior or surface coating from the Cb (or other refractorymetal) of the substrate by the intermediate or interior coating zone,thereby resulting in less quantitative diffusion of the substrate metalinto the exterior coating zone.

If comparably large amounts of Cr were present in the interior coatingzone, it could lead to a competing reaction between Cr and Cb in theinterior coating for production of Cr-aluminides rather than the desiredCbaluminides. For this reason, only the much lesser amounts of Cr setforth above can be present in the interior coating zone. The high levelsof Cr present in the exterior coating zone, however, contributesignificantly to the improved performance of the coatings of thisinvention, particularly at temperatures approaching about 2000 F. Theexterior coating zone in the as-applied condition cannot have a Crcontent greater than 33%, however, because such higher levels tend toproduce an undesirably low level of coating fluidity.

The exterior coating composition is also preferably applied by a coldspray slurry process in the form of a dispersion in a suitablevaporizable lacquer. After this coating composition is applied to thecomposite, which comprises the refractory metal substrate having theabovedescribed interior coating zone adjacent to it and the abovedescribed exterior coating zone superimposed on that interior coatingzone, the composite is again heat treated at from 1800 to 2000" F. forfrom /2 to 16 hours, and preferably from 1.5 to 4 hours, to adherentlyand metallurgically bond the exterior coating zone onto the composite.Again, heat treatment at 1950 F. for about 2 hours is consideredoptimum.

The exterior coating composition is preferably adhered to the compositein an amount of about 20 to 25 mg./ cm. optimum is about 22 mg./cm. ofsurface area, although these amounts are not critical. The resultingproduct is a coated refractory metal article having excellent resistanceto oxidation and contamination at intermediate temperature ranges up toabout 2000 F. for long periods of time.

The overall fired thickness of the two-zone coatings of this inventionis from about 3 mils to about 7 mils, and preferably is about 3 t mils.

"Ti-degradation of the mechanical and strength properties of thesubstrates coated in accordance with this invention is not a problem,because the amounts of Ti present in the coatings are relatively small,and the Ti is normally chemically tied in the coating, as anintermetallic compound or the like, and is not likely to migrate ordiffuse into the substrate in an amount sufficient to cause anysignificant problem of substrate degradation.

Minor amounts of iron (Fe), manganese (Mn), and boron (B) can be presentin the as-applied coatings of this invention without materiallyaffecting coating performance. Total amounts of Fe, Mn, and B in excessof 3%, and amounts of any one of these elements in excess of 1%,however, can be detrimental and should not be used.

It will be understood that whenever parts of percentages are referred toin this specification and in the appended claims, this is intended tomean parts or percentages by weight, unless otherwise specificallyindicated.

The alteration of the structure of the coatings of this invention in useis clearly shown by the photomicrographs FIGS. 1 through 6 whichillustrate a Cb-lZr alloy coated in accordance with this invention andexposed in impure argon at about 2000 F. for times up to 5000 hours. Theargon atmosphere was dynamic with about ten volume changes per hour, andthe argon contained up to 2.5 p.p.m. of oxygen and 5 ppm. of watervapor.

FIG. 1 is a photomicrograph taken in polarized light and enlarged 500times which shows the coated substrate in the as-applied condition, witha Cb Sn zone almost immediately adjacent to the Cb-lZr alloy matrix. TheCbAl zone and the Sn-rich envelope zone are respectively superimposed onthe Cb Sn first coating zone. The envelope zone comprises a Sn-richmatrix containing varying proportions of all of the coating elements.The envelope zone behaves as a reservoir, supplying Sn and Al for thegrowth of the CbAl and Cb Sn phases.

FIG. 2 is a photomicrograph taken in polarized light and enlarged 500times which shows the same coated alloy after exposure in argon forabout 100 hours at 2000" F. FIG. 2 reveals that exposure for this periodproduced some growth of the Cb Sn and CbAl zones at the expense of theenvelope zone. No noticeable increase is evident in the very thinsubaluminide zone (largely Cb Al) located between the Cb Sn zone and thesubstrate.

FIG. 3 is a photomicrograph taken in polarized light and enlarged 500times which shows the coating on the CblZr alloy substrate after about500 hours of exposure in the dynamic argon atmosphere at 2000 F. FIG. 3reveals that after 500 hours there has been no significant growth of thesubaluminide zone, but that there has been significant growth of the CbSn and CbAl zone, again at the expense of the envelope zone. The excessSn and Al of the envelope zone have been largely depleted after 500hours, although the envelope zone has not disappeared entirely at thistime.

FIG. 4 is a photomicrograph taken in polarized light and enlarged 500times which shows the coating of this invention on the Cb-lZr alloysubstrate after exposure in the dynamic argon atmosphere for about 1000hours at 2000 F. The envelope zone has now disappeared completely and anoticeable expansion of the subaluminide zone has occurred.

FIGS. 5 and 6 are photomicrographs taken in polarized light and enlarged500 times which show the coating of this invention on the CblZr alloysubstrate after exposure in an argon atmosphere at 2000" F. for 2500hours and 5000 hours, respectively. These figures show the rapid growthof the subaluminide zone which commenced after disappearance of theenvelope zone. They also show the formation of two phases within thesubaluminide zone, with lower subaluminides (probably Cb Al) beingformed at the substrate-coating interface.

FIG. 6 clearly shows that the rapid growth of the subaluminide zone,after disappearance of the envelope zone, occurs largely at the expenseof the primary oxidation and contamination protective CbAl coating zone.However, it can be seen that this protective layer remained intact andcapable of providing a substantial period of additional protection evenafter 5000 hours of exposure.

The coatings shown in the foregoing photomicrographs were those ofExample 1, which are substantially the optimum coatings of thisinvention.

Each of the coating zones of this invention is preferably applied by acold spray slurry process. This process i readily adaptable to scalingup from use on laboratory specimens to use on dimensionally largeconfigurations Without any sacrifice in coating performance. It is alsoamenable to application of coatings or repairs of coatings in the field.This coating procedure is further desirable in that it does not requireexcessively high temperatures and hence does not produce or contributeto thermal damage or interstitial contamination of the refractory metalsubstrate.

One important advantage of the coatings of this invention is that theycan be applied to complex engineering structures. The coatings have beentested to determine the feasibility of their use for such purposes, andwere found to be entirely satisfactory.

Moreover, the cold slurry spray process is useful in that it can producemulti-component composites of various combinations of a wide variety ofelements and compounds. It also achieves a uniformity in both thicknessand composition of coating from place to plac on the workpiece surface.All that is required for the cold spray slurry process to be effectiveis a clean surface, spray coating or brushing of the slurry onto thearea to be coated with blending into any already coated surfaces. andinertatmosphere heat treatment. The latter may be accomplished usingportable apparatus which have been developed for annealing field welds.

Before coating, the surfaces of the substrate should be thoroughlycleaned of dust, dirt, or other foreign substances. This may beaccomplished by water rinsing, liquid blasting, washing in suitableorganic or inorganic solvents, or immersion in alkali cleaners or acidpickles. Care should b taken in cleaning the substrate to insure removalof all foreign matter.

After the surface has been cleaned, a metal powder mixture of theinterior coating composition is dispersed in a suitable liquid diluent,and the resulting dispersion is applied to the substrate by spraying,brushing, dipcoating, or any other effective method. As pointed outabove, spraying is generally preferred.

The diluent used in the preparation of the dispersion can be anycompatible diluent, Any of the well-known diluents employed with resinsand polymers in the paint industry may be used. Preferably, a readilyvolatilizable organic solvent or mixture of solvents is used. Examplesof solvents that can be used are lower aliphatic alcohols, loweraliphatic ketones, lower alkyl esters or lower aliphatic acids, andlower hydrocarbons such as benzene and lower alkyl substituted benzene.Non-limiting examples of such diluents are methyl, ethyl, propyl, andbutyl alcohols; acetone, methyl ethyl ketone, diethyl ketone, and octylhexyl ketone; methyl acetate, butyl acetate, octyl 1 1 acetate, methylpropionate, octyl hexanoate; benzene, toluene, xylene, ethyl benzene;and the like.

The organic solvents mentioned are illustrative only and are not to beconsidered limiting. The main requirement of the volatile liquidsubstance or diluent is that it be reasonably safe to use, inexpensive,and sufiiciently liquid at ordinary temperatures to act as a dispersantfor the metallic powders so that the dispersion can be sprayed orsuitably coated on the specimen, and at the same time be sufficientlyvolatile to evaporate when exposed to atmospheric or other conditions aswill be described below.

If desired, a binding or sticking agent can be added to the liquiddiluent to hold the powder mixture to the surface of the substrate afterevaporation of the solvent. Use of a binder enables the powders toadhere to the substrate for prolonged periods of time, therebyprecluding the necessity of heat treating immediately after applicationof powder or of taking special precautions in handling the treatedsubstrate to avoid loss of powders.

The binder should be one that will substantially completely decomposeduring heat treatment and that will preferably decompose at atemperature below the melting point of the lowest melting metal orcombination Of metals used. Suitable binders or sticking agents that maybe mentioned include nitrocellulose, naphthalene, and stearates. Othersticking or binding agents will be readily apparent to those skilled inthe art.

Suitable wetting agents may also be added to the diluent if required.Moreover, low boiling organic compounds in small amounts can be added tothe diluent to enhance its rapid evaporation.

In accordance with the invention, a dispersion of the metal powders ofthe interior coating composition, such as, for example, Sn, Al, Cr, Ti,Zn, and UP in a liquid diluent, or preferably in a lacquera diluentcontaining a hinder or sticking agentis deposited on the surface of asubstrate to be coated in the manner already described. Afterapplication the solvent is allowed to evaporate and a mixture of metalpowders is left on the substrate. If a sticking agent is added to thediluent, upon evaporation of the solvent the sticking agent will remaindispersed throughout the powder mixture in the coating, and will serveto hold the powder or dust on th substrate before heat treatment begins.

Evaporation of the volatile solvent, or a volatile portion of thelacquer, containing a sticking agent, may be conveniently brought aboutby allowing the coated substrate to be stored in an atmosphericenvironment at room temperature. If desired, suction or vacuum andelevated temperatures may be used to accelerate evaporation of thvolatile solvent. Evaporation of the solvent leaves a fine layer ofmetallic powder mixture on the surface of the substrate to be heattreated.

The ratio of metallic powders to liquid diluent may vary from about 1:1to l:l volume percent or higher. with the amount of diluent beingadjusted to suit the particular method of application. A ratio of powderto diluent of 1:1 volume percent is satisfactory when it is desired touse a spatula to spread the coating on the surface to be protected. Forspray application, however, the coating composition will be of properconsistency when the ratio of powders to diluent is about 1:10 volumepercent. Still larger amounts of diluent may be used if desired;however, amounts of diluent in excess of a powder to diluent ratio ofabout 1:10 are of no particular advantage and increase the amount ofdiluent that must be evaporated from the coating.

The metallic powders may be mixed in the diluent or lacquer by any ofthe arts well known in the paint industry, or simply by using a WaringBlendor or a ball mill.

A preferred lacquer or diluent with binding or sticking agent for usewith the coating compositions of this invention is nitrocelluloselacquer, i.e., nitrocellulose dissolved in an organic solvent such asamyl acetate.

After the solvent has been allowed to evaporate from the surface of thesubstrate, the resulting specimen is ready for heat treatment in asuitable furnace or oven to complete the formation of the interiorcoating zone on the substrate surface.

The foregoing description of the various suitable diluents and methodsof application for the interior coating composition of this inventionalso apply to the preparation and application of the exterior coatingcomposition. In other words, diluents suitable for use with the interiorcoating composition are also suitable for use with the exterior coatingcomposition.

The metal powders used in the coating compositions of this inventionpreferably have a size range that will permit them to pass through a 200mesh screen, although coarser particles up to a size that will passthrough a mesh screen may also be used. Especially good results areobtained when the size range of metal powders is reduced to a size thatwill pass through a 325 mesh screen (43 microns), or between about 0 to43 microns, and preferably between about 0 to 10 microns. As a generalrule, it can be said that the finer the particles, the better will bethe final coating produced. The mesh sizes referred to above are Tylerstandard.

The use of a fine mesh metal powder helps to keep the powders insuspension and in a slurry and hence is desirable. The larger theparticles are, the more differences in specific gravity of the powdersproduce tendencies to sep aration and make dispersion of the powders inliquid carriers more difficult. Further, as the particle size decreass,the surface area per unit weight increases and reaction is thus promotedby having powders of small particle size.

It should be noted, however, that the above advantages of fine particlesize must be balanced against the increase in oxygen content of thecoating that can result from the use of small particles having a largertotal oxidized surface area.

The metal and halide activator dust or powders can be applied to therefractory metal substrates in any suitable manner. As pointed outabove, the application of these powders in the form of a dispersion in adiluent is preferred. However, a fine film of the powders may be blastedor dusted onto the substrate, or any other suitable means can be used.

A preferred halide activator for use with the second coating compositionis lithium fluoride (LiF). LiF is soluble in many of the organicsolvents mentioned above, and when it is used as the activator, anorganic solvent is selected in which it will readily dissolve.Similarly, when other halides of alkali metals and alkaline earth metalsare used as activators, the particular halide used should be soluble inthe particular organic solvent selected for use as the diluent.

The coatings of this invention are designed to be used on refractorymetal substrates generally. They are most important, however, inproviding desired oxidation and contamination protection to Cb orCb-alloy substrates. It has been found that the beneficial properties ofthe coatings of this invention are most apparent where the substrates towhich these coatings are applied are Cb-base alloys (i.e., alloyscontaining at least 40% Cb) which contain significant amounts (at leastabout 5%) of Ti.

For a clearer understanding of the invention specific examples of it areset forth below. These examples are merely illustrative and are not tobe understood as limiting the scope and underlying principles of theinvention in any way.

EXAMPLE 1 The specimen blanks used in this example were sheared from aCb-lZr alloy sheet stock to a nominal size of 0.625 inch x 0.625 inch x0.030 inch, and a 0.125 inch diameter hole was punched at one end ofeach sample to facilitate handling. The blanks were tumbled in a ballmill using porcelain balls and alumina grit, for 100 hours to relievethe edges of the specimens. The blanks were then etched for minutes inan acid solution consisting of HF, 28% HNO and 62% H O to remove surfacecontamination and were subsequently vacuum heat treated to providestress relief. A typical heat treatment for these Cb1Zr alloy specimenswas 16 hours at 1800 F.

Immediately prior to application of the coating, the specimen blankswere degreased in trichloroethylene vapor, immersed 5 minutes in aheavy-duty alkali cleaning solution at 150 F. rinsed in water, etchedfor 3 additional minutes in the above described acid etching solution atroom temperature, rinsed again in tap water and then in deionized water,dried, and placed on spraying racks.

The interior coating composition was prepared by dry mixing thefollowing high purity metal powders in the proportions indicated:

70% by weight of Sn powder (-325 mesh or finer;

9999+ percent purity),

13 by weight of Al powder (flaked Al powder or finer),

9% by weight of Ti powder (-325 mesh or finer;

99+ percent purity),

4% by weight of Cr powder (-325 mesh or finer;

999+ percent purity),

2% by weight of Zn powder (-325 mesh or finer;

999+ percent purity), and

2% by weight of LiF powder (-325 mesh or finer;

chemically pure grade).

This metal powder mixture was suspended in nitrocellulose lacquer(nitrocellulose dissolved in amyl acetate) by mixing in a WaringBlendor. To provide a quantity of interior coating composition suitablefor spraying, approximately 50 grams of dry powder per 40 cc.s oflacquer were mixed together. After uniformly mixing, the coating wasapplied to the specimen blanks by spraying at a rate of about 23 mg./cm.of surface area. The interior coating composition thus applied had asprayed-on thickness of about 3 mils. This coating composition was thenpermitted to dry in air with the aid of a heat lamp for at least 2 hoursat temperatures up to about 250 F.

At the end of this time substantially all of the organic solvent hadevaporated from the nitrocellulose lacquer leaving a coating compositionon the specimen blanks of metal powders, UP and nitrocellulose as abinder or stick ing agent. The LiF powder dissolved in thenitrocellulose lacquer when the powder mixture was mixed with thelacquer. (Alternatively, the UP powder can be dissolved in the lacquerbefore it is mixed with the metal powders.) When the solvent wasevaporated, the LiF precipitated out and remained substantially evenlydistributed throughout the coating composition.

The specimen was then subjected to heating in an argon atmospherefurnace for 2 hours at a temperature of about 1950 F. The specimens wereinitially heated to about 225 F. in the furnace, at an argon gas flowrate of about 3 to 10 room temperature volume changes per hour, and thenthe temperature of the furnace was rapidly raised to the 1950 F. firingtemperature. In accordance with preferred procedure, the time requiredto reach the firing temperature was less than 3 hours. After this heattreatment the interior coating zone had a thickness of about 2 mils.

Following this heat treatment of the interior coating zone, thespecimens were cleaned by light brushing with a clean stainless steelbrush to remove any powder deposit or other excess material remaining ontheir surfaces and to prepare the surfaces for application of theexterior coating composition.

The exterior coating composition was prepared by mixing high puritymetallic powders in the following proport1ons:

45% by weight of Sn powder (-325 mesh or finer;

9999+ percent purity),

30% by weight of Cr powder (-325 mesh or finer;

99.9-lpercent purity),

16% by weight of Al powder (flaked Al powder or finer),

and

9% by weight of Ti powder (-325 mesh or finer; 99+

percent purity).

This metal powder mixture was also suspended in a nitrocellulose lacquerof the type described above by mixing in a Waring Blendor. To provide aquantity of this second coating composition suitable for spraying,approximately 50 grams of dry powder per 40 ccfs of lacquer were mixedtogether. After mixing, the coating was applied to the specimen blanksby spraying at the rate of about 22 mg./cm. of specimen surface. Theexterior coating composition thus applied had a sprayed-on thickness ofabout 3 mils.

The exterior coating composition was then air dried with the aid of heatlamps for at least 2 hours at temperatures up to about 250 F. Thisresulted in evaporation of substantially all of the organic solvent fromthe coating. The composite was then heat treated in an argon atmospherefurnace, in the manner described above, for a period of /2 hour at 1950F. The resultant article had a two-zone coating thickness of about 3 to4 mils.

Coated specimens produced in the above manner (on Cb-lZr alloysubstrates) were endurance tested to determine their air exposurelifetimes at 650 C. (1200 F.), 871 C. (1600 F.), and 1095 C. (2000 F.).The endurance test specimens were placed on slotted ceramic supports andinserted in preheated furnaces at the testing temperatures. Thespecimens were thermal-cycled to room temperature daily for examinationduring the first 1000 hours, and after that were examined once a week.Failure criterion was the first appearance of Cb-oxide on the specimens.

A total of Cb-lZr alloy specimens coated in the above manner were testedin two groups at 1200 F. in the manner described above. Testing of thefirst group of 42 samples was terminated after 4464 hours, and thesecond group of 48 samples was tested for 10,000 hours. A total of 32 ofthe 42 specimens in the first group had not failed when the test wasended after 4464 hours. Over half of the specimens in the second group(25 out of 48) had not failed after 10,000 hours of exposure, and 39 ofthe 48 second group specimens exhibited coating lifetimes in excess of5000 hours.

A total of 91 Cr-lZr alloy specimens, prepared in the above manner, wereendurance tested by the same procedure in air at 1600" F. Thesespecimens were also thermal-cycled to room temperature once each day forthe first 1000 hours of testing and thereafter once per week. The mediancoating life of these specimens was 1128 hours. With a number ofspecimens, protection for over 4000 hours was obtained, and instances of10,000 hour protection at 1600 F. were also exhibited in this testing.

Four (4) specimens coated in accordance with this invention were exposedto air at 2000 F. to failure. At this temperature, the specimensexhibited coating lifetimes from 144 to 720 hours.

EXAMPLE 2 A number of Cb alloy specimen blanks consisting essentially of1% by weight of Zr, 0.1% by weight of carbon, and balance essentiallyall Cb were prepared and coated in accordance with the procedure setforth in Example 1. Both interior and exterior coating compositionshaving the same ingredients as in Example 1 were applied to the Cb-basealloy specimen blanks in the same manner set forth in Example 1, withheat treatment after the application of each coating, in the mannerdescribed in that example. This procedure produced a metal articlehaving a coating of the desired properties on the surface of theCb-lZr-0.1C alloy substrate.

EXAMPLE 3 Eightly-five (85) Cb-base alloy specimen blanks consistingessentially of 8% by weight of Ti, 4% by weight of Mo, and balanceessentially all Cb, were coated in accordance with the procedure setforth in Example 1. Interior and exterior coating compositions of thesame ingredients as in Example 1 were applied to the Cb-base alloyspecimen blanks in the same manner set forth in Example 1, with heattreatment of the character described in that example following theapplication of each coating composition. The resulting product had acoating of the desired properties on the surface of the Cb-8Ti-4Mo alloysubstrate.

Some 42 of these coated specimen blanks were tested for coatingendurance life in air at 1200 F. in the same manner set forth inExample 1. None of the 42 specimens had failed when the test wasterminated after 4464 hours.

The remaining 43 specimen blanks were endurance tested in air at 1600F., also in the manner set forth in Example 1. The median coating lifeof these specimens at 1600 F. was 3240 hours and 40 of the 43 specimensexhibited a coating life of at least 2400 hours.

EXAMPLE 4 A Cb-base alloy specimen blank consisting essentially of 15%by weight of Ti, 3% by weight of Al, and balance essentially all Cb wasprepared in accordance with the procedures set forth in Example 1.Interior and exterior coating compositions of the same ingredients as inExample 1 were applied to the Cb-base alloy specimen in the same mannerset forth in Example 1, with heat treatment following the application ofeach coating composition, in accordance with the procedure of thatexample. The resulting product had a coating of the desired propertieson the surface of the Cb-lSTi-3Al alloy substrate.

A total of 94 of these coated specimens were subjected to oxidationexposure testing in accordance with the procedure of Example 1. Of the47 specimens tested at 1200 F., 20 were unfailed when the testing wasconcluded after 10,000 hours. The median oxidation endurance lifetime ofthese specimens at 1200 F. was 9312 hours, and 40 of the specimensreached at least 6000 hours before failure.

The 47 specimens tested at 1600 F., also in accordance with theprocedure of Example 1, exhibited a median coating life of 960 hours.

EXAMPLE A Cb-base alloy specimen blank consisting essentially by weightof Ti, 5% V, and balance essentially all Cb was prepared in accordancewith the procedures set forth in Example 1. Interior and exteriorcoating compositions of the same ingredients as in Example 1 wereapplied to the Cb-base alloy specimen in the same manner set forth inExample 1 with heat treatment, in the manner set forth in that example,following the application of each of the coating compositions. Theresultant product had a coating of the desired properties formed on theCb15Ti-5V alloy substrate.

Ninety-one (91) specimen blanks prepared in accordance with this examplewere oxidation endurance tested in accordance with the procedure ofExample 1. The fortytwo (42) specimens tested at 1200 F. exhibited amedian coating life of 8688 hours, and 17 of these 42 specimens wereunfailed when the testing was concluded after 10,000 hours.

The forty-nine (49) specimens tested at 1600 F. exhibited a mediancoating life of 456 hours, with 21 of the specimens affording protectionfor over 1125 hours.

EXAMPLE 6 A Cb-base alloy specimen blank consisting essentially of byweight of Ti and balance essentially all Cb was prepared in accordancewith the procedures set forth in Example 1. Interior and exteriorcoating compositions of the same ingredients as in Example 1 wereapplied to the Cb-base alloy specimen, in the manner set forth inExample 1, with each coating being heat treated on the substratefollowing its application, in accordance with the procedures ofExample 1. The resulting product had a coating of the desired propertiesformed on the surface of the Cb-2OTi alloy substrate.

Thirty-five (35) specimens prepared in accordance with this example wereoxidation endurance tested by the procedure of Example 1.

The eighteen (18) specimens tested at 1200 F. were all unfailed when thetesting was concluded after 4464 hours; and the seventeen (17) specimenstested at 1600 F. exhibited a median coating life of 2400 hours.

EXAMPLE 7 A Ta-base alloy specimen blank consisting essentially of 8% byweight of W, 2% by Weight of Hf, and balance essentially all Ta wasprepared in accordance with the procedures set forth in Example 1.Interior and exterior coating compositions of the same ingredients as inExample 1 were applied to this Ta-base alloy specimen in the same manneras set forth in Example 1, with heat treatment of each coatingcomposition following its application to the specimen, in accordancewith the procedure of that example. The resulting product had a coatingof the desired properties formed on the surface of the Ta- SW-ZHf alloysubstrate.

EXAMPLE 8 A Mobase alloy specimen blank consisting essentially of 0.5%by weight of Ti, 0.08% by weight of Zr, 0.02% by weight of C, andbalance essentially all Mo was prepared in accordance with the procedureset forth in Example l. Interior and exterior coating compositions ofthe same ingredients as in Example 1 were applied to this Mobase alloyspecimen in the same manner as set forth in Example 1, with heattreatment of each coating composition following its application to thespecimen, in accordance with the procedure of that example. Theresulting product had a coating of the desired properties formed otn thesurface of the Mo-0.5Ti0.08Zr-0.02C alloy subs rate.

EXAMPLE 9 A tungsten specimen blank consisting essentially of pure W wasprepared in accordance with the procedures set forth in Example 1.Interior and exterior coating compositrons of the same ingredients as inExample 1 were applied to this W specimen in the same manner as setforth in Example 1, with heat treatment of each coating compositionfollowing its application to the specimen, in accordance with theprocedure of that example. The resulting product had a coating of thedesired properties formed on the surface of the W substrate.

The invention in its broader aspects is not limited to the specificdetails shown and described, but departures may be made from suchdetails within the scope of the accompanyrng claims without departingfrom the principles pf the invention and without sacrificing its chiefadvanages.

What is claimed is:

1. A metal article comprising a refractory metal or a refractorymetal-base alloy substrate, said refractory metal being selected fromthe group consisting of columbium, tantalum, molybdenum and tungsten,and an oxidation and contamination resistant coating on the substrate,the coating comprising an interior coating zone superimposed on andmetallurgically bonded to the substrate, and an exterior coating zonesuperimposed on and metallurgically bonded to the interior coating zone,the interror coating zone consisting essentially of the refractory metalor refractory metal alloy of the substrate, which diffuses into theinterior coating zone during the metallurgical bonding of the interiorcoating zone to the substrate,

and an interior coating composition which consists essentially, byweight, of:

from 65 to 75% of Sn,

from 11 to 15% of A1,

from 7 to 11% of Ti,

from 2 to 6% of Cr,

to 4% of Zn, and

0 to 3% of at least one alkali metal halide or alkaline earth metalhalide; and the exterior coating zone consisting essentially of therefractory metal or refractory metal alloy of the substrate, whichdiffuses into the exterior coating zone during metallurgical bonding ofthe exterior coating zone to the interior coating zone, and exteriorcoating composition which consists essentially of:

from 40 to 50% of Sn,

from 27 to 33% of Cr,

from 14 to 18% of Al, and

from 7 to 11% of Ti.

2. The article of claim 1 in which the interior coating compositioncontains from 1 to 4% by weight of Zn, and from 1 to 3% by weight of atleast one alkali metal halide or alkaline earth metal halide.

3. The article of claim 1 in which the substrate comprises Cb or aCb-base alloy.

4. The article of claim 3 in which the substrate consists essentially ofabout 1% by weight of Zr and the balance essentially Ch.

5. The article of claim 3 in which the substrate consists essentially ofabout 1% by weight of Zr, about 0.1% by weight of C and balanceessen'tially Cb.

6. The article of claim 3 in which the substrate comprises a Cb-basealloy which contains at least about 5% by weight of Ti.

7. The article of claim 6 in which the substrate consists essentially ofabout 8% by weight of Ti, about 4% by weight of Mo, and balanceessentially Ch.

8. The article of claim 6 in which the substrate consists essentially ofabout 15% by weight of Ti, about 3% by weight of Al, and balanceessentially Ch.

9. The article of claim 6 in which the substrate consists essentially ofabout by weight of Ti, about 5% by weight of V, and balance essentiallyCb.

10. The article of claim 6 in which the substrate consists essentiallyof about by weight of Ti and balance essentially Ch.

11. The article of claim 1 in which the substrate comprises Ta or aTa-base alloy.

12. The article of claim 11 in which the substrate consists essentiallyof about 8% by weight of W, about 2% by weight of Hf, and balanceessentially Ta.

13. The article of claim 1 in which the substrate comprises M0 or aMobase alloy.

14. The article of claim 1 in which the substrate consists essentiallyof about 0.5% by weight of Ti, about 0.08% by weight of Zr, about 0.02%by weight of C, and balance essentially Mo.

15. The article of claim 1 in which the substrate comprises W or aW-base alloy.

16. The article of claim 1 in which the interior coating compositionconsists essentially, by weight, of about 70% Sn, about 13% Al, about 9%Ti, about 4% Cr, about 2% Zn, and about 2% HF; and the exterior coatingcomposition consists essentially, by weight, of about 45% of Sn, about16% of Al, about 9% of Ti, and about of Cr.

17. The article of claim 16 in which the substrate comprises Cb or a Cbbase alloy.

References Cited UNITED STATES PATENTS 3,078,554 2/1963 Carlson 291943,216,806 ll/1965 Santa 29198 X 3,360,350 12/1967 Sama 29-498 X HYLANDBlZO'l, Primary Examiner US. or. x11, 29 19s

